Solids have definite volume and shape.
solids can be classified into the following two major types based
on the arrangement of their constituents. (i) Crystalline solids (ii)Amorphous
A crystalline solid is one in which its constituents (atoms, ions
or molecules), have an orderly arrangement extending over a long range.
In contrast, in amorphous solids (In Greek, amorphous means no
form) the constituents are randomly arranged.
Crystalline solid is characterised by a definite orientation of
atoms, ions or molecules, relative to one another in a three dimensional
pattern. The regular arrangement of these species throughout the crystal is
called a crystal lattice.
A crystal may be considered to consist of large number of unit
cells, each one in direct contact with its nearer neighbour and all similarly
oriented in space.
A unit cell is characterised by the three edge lengths or lattice
constants a ,b and c and the angle between the edges α, β and γ
There are seven primitive crystal systems; cubic, tetragonal,
orthorhombic, hexagonal, monoclinic, triclinic and rhombohedral. They differ in
the arrangement of their crystallographic axes and angles. Corresponding to the
above seven, Bravis defined 14 possible crystal systems
In the simple cubic unit cell, each corner is occupied by an
identical atoms or ions or molecules. And they touch along the edges of the
cube, do not touch diagonally. The coordination number of each atom is 6.
In a body centered cubic unit cell, each corner is occupied by an
identical particle and in addition to that one atom occupies the body centre.
Those atoms which occupy the corners do not touch each other, however they all
touch the one that occupies the body centre. Hence, each atom is surrounded by
eight nearest neighbours and coordination number is 8.
In a face centered cubic unit cell, identical atoms lie at each
corner as well as in the centre of each face. Those atoms in the corners touch
those in the faces but not each other. The coordination number is 2.
X-Ray diffraction analysis is the most powerful tool for the
determination of crystal structure. The inter planar distance (d) between two
successive planes of atoms can be calculated using the following equation form
the X-Ray diffraction data 2dsinθ
The structure of an ionic compound depends upon the stoichiometry
and the size of the ions. generally in ionic crystals the bigger anions are
present in the close packed arrangements and the cations occupy the voids. The
ratio of radius of cation and anion (rC/rA) plays an
important role in determining the structure
Crystals always found to have some defects in the arrangement of
their constituent particles.
Schottky defect arises due to the missing of equal number of
cations and anions from the crystal lattice.
Frenkel defect arises due to the dislocation of ions from its
crystal lattice. The ion which is missing from the lattice point occupies an
Metal excess defect arises due to the presence of more number of
metal ions as compared to anions.
Metal deficiency defect arises due to the presence of less number
of cations than the anions.